1. Field of the Invention
The present invention relates to electromagnets and, more particularly, to an attachment device for connecting an electromagnet to a lifting device such as a crane, whereby sudden loads imparted to the crane by the electromagnet are cushioned.
2. Description of the Prior Art
A typical electromagnet includes a disc-like element having a work-engaging face and a non-work-engaging face oppositely disposed. A plurality of chains or other force-transmitting members are connected at one end to the non-work-engaging face. The chains are connected at the other end to a ring, a clevis, or other connecting member. In turn, a lifting device such as a crane having a hook can be connected to the connecting member. Thereafter, upon raising or lowering the crane, the electromagnet can be moved from place to place.
The type of electromagnet attachment referred to is mechanically simple and reliable, but it has failed to address certain problems. One of these problems relates to the use of electromagnets with hydraulic cranes. A hydraulic crane is a self-propelled device employing a plurality of interconnected booms. Because the electromagnet is directly connected to the outermost boom of the crane, structural problems can develop when sudden loads are imparted from the electromagnet to the crane. These loads can be very severe and can lead to stress cracks, broken pins and bushings, and other maintainence problems in the crane not always identified with the use of electromagnets. It is believed that a resilient attachment between the crane and the electromagnet would solve many of these problems.
Unfortunately, known resilient attachments for connecting electromagnets to cranes have not been adequate for properly absorbing loads imparted to the crane by the electromagnet. For example, one known resilient support for electromagnets employs a ring connected to a plurality of chains, the chains being connected at their other end to the non-work-engaging face of the electromagnet. A tube extends downwardly from the ring toward the upper surface of the non-work-engaging face. A spring disposed within the tube constantly biases the ring upwardly, away from the non-work-engaging face. By this construction, the chains are always maintained in a taut condition. Because the chains are in a taut condition, the ring is maintained in a fixed position relative to the electromagnet. This makes it easier for a crane or other lifting device to be connected to the ring. Unfortunately, it does nothing to absorb loads imparted to the crane upon lifting the electromagnet and moving it from place to place.
Another known attachment for connecting electromagnets to cranes is adapted especially for use with coils of strip steel. The electromagnet includes an opening at its center through which a vertically oriented plunger extends. The lower, central portion of the electromagnet includes a plurality of pivotally mounted fingers. The plunger is disposed at the center of the fingers and is configured such that, upon attaching a lifting device to the plunger and lifting the plunger upwardly, the fingers are urged outwardly. By positioning the fingers and plunger within the center of a coil of strip steel lying on its side, and by lifting the plunger with a crane, the fingers will be urged into tight contact with the inner diameter of the coil. The contact between the fingers and the coil, combined with the attractive force generated by the electromagnet, will be sufficient to enable the coil to be lifted.
A spring is provided in order to cause the plunger to be moved downwardly upon removing force from the crane. The spring is disposed intermediate the underside of a portion of the electromagnet and the upper surface of a portion of the plunger. Although the spring arguably cushions loads imparted to the crane during initial phases of a lifting operation, no resilient support is provided once the fingers have engaged the inner diameter of the coil. In effect, the spring provides no load-absorbing capabilities under virtually all operating conditions.